This invention relates to use of stabilized unsymmetrical coupling agents for use in conjugated diene-based rubber compositions reinforced with silica-based reinforcement, namely use of stabilized 2-benzothiazyl-3-(triethoxysilylpropyl) disulfide and 3-(triethoxysilyl)propyl-N-maleamic acid. Said stabilized unsymmetrical coupling agent materials may be provided for mixing with said diene-based rubber(s), for example, as individual materials, as at least one of said materials being pre-reacted with aggregates of precipitated silica, said precipitated silica having hydroxyl groups (e.g. silanol groups) on its surface. The invention includes an article of manufacture, including a tire, having at least one component comprised of such rubber composition.
For various applications utilizing rubber compositions, carbon black and/or synthetic amorphous silica-based materials are conventionally used as particulate reinforcing fillers for the elastomer(s) of the rubber composition(s).
Often a coupling agent is used together with the silica in order to couple, or otherwise enhance, its elastomer reinforcement effect for the elastomer(s) and such use of a coupling agent is well known to those having skill in such art.
Historically, such coupling agents are composed, for example, of one moiety (e.g. a silane-based moiety such as, for example, an alkoxysilane moiety) which is reactive with hydroxyl groups (e.g. silanol groups) on the surface of a synthetic amorphous silica and another moiety (e.g. a polysulfide bridge) which becomes interactive with a conjugated diene-based elastomer. Typically, such coupling agents are symmetrical compounds.
For example, bis-(3-trialkoxysilylpropyl) polysulfides having an average of from 2 to 4 connecting sulfur atoms in their polysulfidic bridge are often used as coupling agents to enhance coupling of synthetic amorphous silicas, such as for example, aggregates of precipitated silica, to various conjugated diene-based elastomers. Use of such symmetrical coupling agents is well known to those having skill in such art.
However, use of unsymmetrical coupling agents can represent problems not normally encountered by use of the aforesaid symmetrical coupling agents.
For example, various unsymmetrical coupling agents may tend to pre-react with themselves before mixing with a blend of an elastomer and precipitated silica, with their intended use of a coupling agent being seriously impaired if not altogether nullified by the relative short shelf life of the unsymmetrical coupling agents.
Exemplary of unsymmetrical coupling agents contemplated herein are 2-benzothiazyl-3-(triethoxysilylpropyl) disulfide and 3-(triethoxysilyl)propyl-N-maleamic acid. Such unsymmetrical coupling agents have a relatively short shelf life since they inherently tend to react with themselves or with each other if used in combination and therefore have a limited use, in practice, as coupling agents for manufacturing products on an industrial scale.
For example, 2-benzothiazyl-3-(triethoxysilylpropyl) disulfide can proceed to react with itself and form 2-mercaptobenzothiozole and symmetrical disulfides, thereby severely limiting its use as a silica coupling agent.
For example, 3-(triethoxysilyl)propyl-N-maleamic acid can proceed to react with itself to liberate ethanol and form an insoluble polymeric siloxane network, thereby severely limiting its use as a silica coupling agent.
In the description of this invention, the term xe2x80x9cphrxe2x80x9d is used to designate parts by weight of a material per 100 parts by weight of elastomer. In the further description, the terms xe2x80x9crubberxe2x80x9d and xe2x80x9celastomerxe2x80x9d may be used interchangeably unless otherwise mentioned. The terms xe2x80x9cvulcanizedxe2x80x9d and xe2x80x9ccuredxe2x80x9d may be used interchangeably, as well as xe2x80x9cunvulcanizedxe2x80x9d or xe2x80x9cuncuredxe2x80x9d, unless otherwise indicated.
In accordance with one aspect of this invention, a rubber composition is provided which comprises
(A) 100 parts by weight of at least one diene hydrocarbon based elastomer,
(B) about 25 to about 100, optionally about 35 to about 90, phr of particulate reinforcing filler comprised of about 25 to about 100, alternately about 35 to about 90, phr of synthetic amorphous silica aggregates and, correspondingly, from zero to about 75, alternately about 10 to about 65, phr of at least one of carbon black and silica treated carbon black having domains of silica on its surface, wherein said silica aggregates and said silica domains on the surface of said treated carbon black contain hydroxyl groups (e.g. silanol groups) on their surface; and
(C) about 1 to about 100, alternately about 8 to about 15, phr of a pre-treated silica as aggregates of precipitated silica which contain hydroxyl groups (e.g. silanol groups) on their surface which have been reacted with:
(1) 2-benzothiazyl-3-(triethoxysilylpropyl) disulfide, or
(2) 3-(triethoxysilyl)propyl-N-maleamic acid.
A significant aspect of this invention is the use of a composite of a pre-treated silica by reaction with the 2-benzothiazyl-3-(triethoxysilylpropyl) disulfide or 3-(triethoxysilyl)propyl-N-maleamic acid.
For example, by reaction of the triethoxysilylpropyl moiety of the 2-benzothiazyl-3-(triethoxysilylpropyl) disulfide with the hydroxyl groups of the precipitated silica, its degradation and associated formation of 2-mercaptobenzothiazole and symmetrical disulfides is substantially retarded and the resulting composite of pre-treated silica is readily available for use as a coupling agent for coupling precipitated silica to a diene-based elastomer.
For example, by reaction of the triethoxysilyl moiety of the 3-(triethoxysilyl)propyl-N-maleamic acid with the hydroxyl groups of the precipitated silica, its degradation and associated formation of insoluble polymeric siloxane network is substantially retarded and the resulting composite of pre-treated silica is readily available for use as a coupling agent for coupling precipitated silica to a diene-based elastomer.
In practice, the said composite of pre-treated silica is preferably comprised of about 1 to about 20 percent by weight by weight of the precipitated silica and, correspondingly, about 5 to about 15 weight percent derived from reaction of said 2-benzothiazyl-3-(triethoxysilylpropyl) disulfide or 3-(triethoxysilyl)propyl-N-maleamic acid.
In further accordance with this invention, an article of manufacture is provided which contains at least one component comprised of the rubber composition of this invention.
In additional accordance with this invention, a tire is provided which contains at least one component comprised of the rubber composition of this invention.
In further accordance with this invention, a tire is provided having a tread comprised of the rubber composition of this invention.
It is to be appreciated that additional coupling agents may be used in the practice of this invention to aid in coupling the silica to elastomer(s). Such coupling agents would conventionally have a moiety reactive with hydroxyl groups on the silica (e.g. silanol groups) and another moiety interactive with one or more elastomers. Use of such coupling agents for such purpose is well known to those having skill in such art.
For example, a coupling agent as a bis-(3-trialkoxysilylalkyl) polysulfide having from 2 to 8, with an average of from 2 to 2.6 or from 3.5 to 4, connecting sulfur atoms in its polysulfidic bridge might be used.
Representative of such coupling agent is bis-(3-triethoxysilylpropyl) polysulfide having from 2 to 8, with an average of from 2 to 2.6 or from 3.5 to 4, connecting sulfur atoms in its polysulfidic bridge.
The aggregates of precipitated silica is intended to include precipitated aluminosilicates as a co-precipitated silica with a minor amount of aluminum.
Such precipitated silica is, in general, well known to those having skill in such art. For example, such precipitated silica may be precipitated by controlled addition of an acid such as, for example, hydrochloric acid or sulfuric acid, to a basic solution (e.g. sodium hydroxide) of a silicate, for example, sodium silicate, usually in the presence of an electrolyte, for example, sodium sulfate, which can contain, if desired, a minor amount of aluminum salt. Primary, colloidal silica particles typically form during such process which quickly coalesce to form aggregates of such primary particles and which are then recovered as precipitates by filtering, washing the resulting filter cake with water or an aqueous solution, and drying the recovered precipitated silica. Such method of preparing precipitated silica, and variations thereof, are well known to those having skill in such art.
The carbon black with domains of exposed silica on its surface may be, for example, carbon black prepared by treatment of carbon black by reaction of an alkyl silane (e.g. an alkoxy silane) with carbon black or by co-fuming carbon black and silica at an elevated temperature. For example, see U.S. Pat. Nos. 5,679,728 and 6,028,137.
The 2-benzothiazyl-3-(triethoxysilylpropyl) disulfide or 3-(triethoxysilyl)propyl-N-maleamic acid may be reacted with aggregates of precipitated silica which contain hydroxyl groups on their surface to form composites thereof by, for example, by dissolving the material in a volatile organic solvent such as, for example acetone, and adding the solvent solution to the particulate silica then removing the solvent.
It is envisioned herein that the resulting composite can be mixed with a suitable diene-based elastomer and the silica coupled to the elastomer by interaction in situ within the elastomer host of the mercaptopropyl radical or maleamic acid/maleimide moiety component of the composite, as the case may be.
By the practice of this invention, such in situ reaction within the elastomer host is accomplished without an appreciable, and preferably in the absence of, evolution of an alcohol. Indeed, by the inherent chemistry of the pre-treatment of the precipitated silica prior to addition to the elastomer, any evolution of alcohol as a result of reaction of the hydroxyl groups on the surface of the silica occurs prior to addition of the pre-treated silica to the mixing with the elastomer host.
In another aspect of the invention, the precipitated silica is pre-hydrophobated, at least to some extent, in the following manner, albeit in a low level as to not interfere with the trialkoxy silane reaction of the unsymmetrical coupler with the silica surface.
Thus, the precipitated silica is pre-hydrophobated prior or subsequent to its addition to the unsymmetrical coupler before introduction to the elastomer. This is considered herein to be important as to both the process of mixing and reacting of the mercapto/maleamic acid, as the case may be, moiety of the unsymmetrical coupler.
In particular, it is considered herein that pre- or post-hydrophobation of the silica material, particularly with an alkylalkoxy or halosilane, as the case may be, enables a more efficient mixing, or processing, of it within the elastomer host in that such pre-hydrophobation of the silica material
(A) renders it more compatible with the diene-based elastomer and
(B) substantially reduces a tendency for a precipitated silica to re-agglomerate with itself within the elastomer host.
Moreover, the accompanying in situ reaction of the mercapto or maleamic acid/maleamide moiety of the unsymmetrical coupler now enables both the more efficient mixing coupled with the associated reaction with only a minimal evolution of an alcohol, preferably no evolution of alcohol from the rubber composition.
For some applications, it might be desirable to use a pre-hydrophobated silica which might be beneficial because a pre-hydrophobated silica renders such silica more compatible with the diene-based elastomer and substantially reduces a tendency for a precipitated silica to re-agglomerate with itself within the elastomer host.
Moreover, use of the silica material having been pre-treated with both the unsymmetrical coupler and a hydrophobating agent might be desirable in order to enable both the more efficient mixing of the silica material with the associated diene-based elastomer(s) coupled with the associated reaction of the pre-treated silica of this invention within the elastomer host with only a minimal evolution of an alcohol, preferably no evolution of alcohol from the rubber composition.
The hydrophobated precipitated silica aggregates might be recovered, for example, from treated colloidal silica, for example as a treated silica hydrosol, with the aid of acid addition to the treated colloidal silica (for example, sulfuric acid or hydrochloric acid) followed by water washing and drying the recovered hydrophobated silica as a hydrophobated silica gel or as a hydrophobated precipitated silica. While this invention is not intended to be directed to a specific preparation technique (preparation of silica hydrosols, recovery of silica gels and precipitated silicas, etc.) of the pre-hydrophobated precipitated silica itself, for education purposes in this regard, reference might be made to the Encyclopedia of Chemical Technology, Fourth Edition (1997), Volume 21, Kirk-Othmer in Pages 1020 through 1026 and U.S. Pat. No. 5,094,829 as well as U.S. Pat. Nos. 5,708,069, 5,789,514 and 5,750,610 for a more detailed discussion.
Such pre-hydrophobated silica (a hydrophobation of the silica before mixing it with the diene-based elastomers) may be prepared by reacting the silica (e.g. aggregates of a precipitated silica) with an alkylsilane of the general Formula (III), and optionally an organomercaptosilane of the general formula (IV), with
(A) said aggregates of synthetic precipitated silica or
(B) an aqueous dispersion of colloidal silica particles from which a precipitated silica is recovered to form a silica composite thereof;
wherein said alkylsilane of the general Formula (III) is represented by
xe2x80x83Xnxe2x80x94Sixe2x80x94R4xe2x88x92nxe2x80x83xe2x80x83(III)
wherein R is an alkyl radical having from one to 18 carbon atoms, n is a value of from 1 to 3 and X is a radical selected from chlorine or bromine or alkoxy radical as (OR1)xe2x80x94, wherein R1 is an alkyl radical selected from methyl and ethyl radicals, and where said organomercaptosilane is of the general formula (IV):
(X)n(R2O)3xe2x88x92nxe2x80x94Sixe2x80x94R3xe2x80x94SHxe2x80x83xe2x80x83(IV)
wherein X is a radical selected from chlorine, bromine, and alkyl radicals having from one to 16 carbon atoms; wherein R2 is an alkyl radical having from one to 16 carbon atoms and R3 is an alkylene radical having from one to 16 carbon atoms; and n is a value from zero to 3.
Representative alkylsilanes of Formula (III) are, for example, trichloro methyl silane, dichloro dimethyl silane, chloro trimethyl silane, trimethoxy methyl silane, dimethoxy dimethyl silane, methoxy trimethyl silane, trimethoxy propyl silane, trimethoxy octyl silane, trimethoxy hexadecyl silane, dimethoxy dipropyl silane, triethoxy methyl silane, triethoxy propyl silane, triethoxy octyl silane, and diethoxy dimethyl silane.
In practice, various diene-based elastomers may be used such as, for example, homopolymers and copolymers of monomers selected from isoprene and 1,3-butadiene and copolymers of at least one diene selected from isoprene and 1,3-butadiene and a vinyl aromatic compound selected from styrene and alphamethyl styrene, preferably styrene.
Representative of such conjugated diene-based elastomers are, for example, cis 1,4-polyisoprene (natural and synthetic), cis 1,4-polybutadiene, styrene/butadiene copolymers (aqueous emulsion polymerization prepared and organic solvent solution polymerization prepared), medium vinyl polybutadiene having a vinyl 1,2-content in a range of about 15 to about 90 percent, isoprene/butadiene copolymers, styrene/isoprene/butadiene terpolymers. Tin coupled elastomers may also be used, such as, for example, tin coupled organic solution polymerization prepared styrene/butadiene co-polymers, isoprenelbutadiene copolymers, styrenelisoprene copolymers, polybutadiene and styrene/isoprene/butadiene terpolymers.
In the further practice of this invention, particulate reinforcement for the rubber composition may be particulate synthetic amorphous silica, or a combination of carbon black and amorphous silica (exclusive of silica treated carbon black), usually of an amount in a range of about 35 to about 100 alternately about 35 to about 90, phr. If a combination of such carbon black and silica is used, usually at least about 5 phr of carbon black and at least 10 phr of silica are used. For example, a weight ratio of silica to carbon black ranging from about 1/5 to 5/1 might be used. Commonly employed synthetic amorphous silica, or siliceous pigments, used in rubber compounding applications can be used as the silica in this invention, including precipitated siliceous pigments and fumed (pyrogenic) silica wherein aggregates of precipitated silicas are usually preferred.
The precipitated silica aggregates preferably employed in this invention are precipitated silicas such as, for example, those obtained by the acidification of a soluble silicate, e.g., sodium silicate and may include coprecipitated silica and a minor amount of aluminum.
Such silicas might usually be characterized, for example, by having a BET surface area, as measured using nitrogen gas, preferably in the range of about 40 to about 600, and more usually in a range of about 50 to about 300 square meters per gram. The BET method of measuring surface area is described in the Journal of the American Chemical Society, Volume 60, Page 304 (1930).
The silica may also be typically characterized by having a dibutylphthalate (DBP) absorption value in a range of about 50 to about 400 cm3/100 g, and more usually about 100 to about 300 cm3/100 g.
Various commercially available precipitated silicas may be considered for use in this invention such as, only for example herein, and without limitation, silicas from PPG Industries under the Hi-Sil trademark with designations Hi-Sil 210, Hi-Sil 243, etc; silicas from Rhodia as, for example, Zeosil 1165MP and Zeosil 165GR, silicas from Degussa AG with, for example, designations VN2 and VN3, as well as other grades of silica, particularly precipitated silicas, which can be used for elastomer reinforcement.
It is readily understood by those having skill in the art that the rubber composition would be compounded by methods generally known in the rubber compounding art, such as mixing the various sulfur-vulcanizable constituent rubbers with various commonly used additive materials such as, for example, curing aids, such as sulfur, activators, retarders and accelerators, processing additives, such as oils, resins including tackifying resins, silicas, and plasticizers, fillers, pigments, fatty acid, zinc oxide, waxes, antioxidants and antiozonants, peptizing agents and reinforcing materials such as, for example, carbon black. As known to those skilled in the art, depending on the intended use of the sulfur vulcanizable and sulfur vulcanized material (rubbers), the additives mentioned above are selected and commonly used in conventional amounts.
Typical amounts of tackifier resins, if used, comprise about 0.5 to about 10 phr, usually about 1 to about 5 phr. Typical amounts of processing aids comprise about 1 to about 50 phr. Such processing aids can include, for example, aromatic, napthenic, and/or paraffinic processing oils. Typical amounts of antioxidants comprise about 1 to about 5 phr. Representative antioxidants may be, for example, diphenyl-p-phenylenediamine and others, such as, for example, those disclosed in The Vanderbilt Rubber Handbook (1978), Pages 344 through 346. Typical amounts of antiozonants comprise about 1 to 5 phr. Typical amounts of fatty acids, if used, which can include stearic acid comprise about 0.5 to about 3 phr. Typical amounts of zinc oxide comprise about 1 to about 10 phr. Typical amounts of waxes comprise about 1 to about 5 phr. Often microcrystalline waxes are used. Typical amounts of peptizers comprise about 0.1 to about 1 phr.
The vulcanization is conducted in the presence of a sulfur vulcanizing agent. Examples of suitable sulfur vulcanizing agents include elemental sulfur (free sulfur) or sulfur donating vulcanizing agents, for example, an amine disulfide, polymeric polysulfide or sulfur olefin adducts. Preferably, the sulfur vulcanizing agent is elemental sulfur. As known to those skilled in the art, sulfur vulcanizing agents are used in an amount ranging from about 0.5 to about 4 phr, or even, in some circumstances, up to about 8 phr.
Accelerators are used to control the time and/or temperature required for vulcanization and to improve the properties of the vulcanizate. In one embodiment, a single accelerator system may be used, i.e., primary accelerator. Conventionally and preferably, a primary accelerator(s) is used in total amounts ranging from about 0.5 to about 4, preferably about 0.8 to about 1.5, phr. In another embodiment, combinations of a primary and a secondary accelerator might be used with the secondary accelerator being used in smaller amounts (of about 0.05 to about 3 phr) in order to activate and to improve the properties of the vulcanizate. Combinations of these accelerators might be expected to produce a synergistic effect on the final properties and are somewhat better than those produced by use of either accelerator alone. In addition, delayed action accelerators may be used which are not affected by normal processing temperatures but produce a satisfactory cure at ordinary vulcanization temperatures. Vulcanization retarders might also be used. Suitable types of accelerators that may be used in the present invention are amines, disulfides, guanidines, thioureas, thiazoles, thiurams, sulfenamides, dithiocarbamates and xanthates. Preferably, the primary accelerator is a sulfenamide. If a second accelerator is used, the secondary accelerator is preferably a guanidine, dithiocarbamate or thiuram compound.
The presence and relative amounts of the above additives are not considered to be an aspect of the present invention, unless otherwise indicated herein, which is more primarily directed to the utilization a non-silane containing coupling agent for enhancing reinforcement of a conjugated diene-based elastomer with a particulate synthetic amorphous silica material which contains hydroxyl groups (e.g. silanol groups) on its surface.
The mixing of the rubber composition can be accomplished by methods known to those having skill in the rubber mixing art. For example, the ingredients are typically mixed in at least two stages, namely, at least one non-productive stage followed by a productive mix stage. The final curatives are typically mixed in the final stage which is conventionally called the xe2x80x9cproductivexe2x80x9d mix stage in which the mixing typically occurs at a temperature, or ultimate temperature, lower than the mix temperature(s) than the preceding non-productive mix stage(s). The rubber, and fillers such as silica and silica treated carbon black and adhesive agent, are mixed in one or more non-productive mix stages. The terms xe2x80x9cnon-productivexe2x80x9d and xe2x80x9cproductivexe2x80x9d mix stages are well known to those having skill in the rubber mixing art.